1. Field of the Invention
The present invention relates to a sequential mesa avalanche photodiode and a method of manufacturing the same, and in particular, to a sequential mesa avalanche photodiode having a sequential mesa structure in which, in an avalanche photodiode to be used as a light receiving element for converting a light signal to an electric signal in an optical communication network or the like, high sensitization can be realized and the fabrication costs of modularization can be greatly decreased, and to a method of manufacturing the same.
2. Description of the Related Art
As is well-known, recently, the signal speed of light signals used in optical communication networks has been made much more high-speed.
In accordance therewith, making the speed more high-speed has been required of light receiving elements built in optical communication equipment transmitting and receiving such light signals.
Further, in such light receiving elements, it is required that even low level light signals can be precisely received.
As such a light receiving element receiving high-speed and weak light signals, generally, an avalanche photodiode (hereinafter, abbreviated APD) has been put into practice.
In such an APD, in a state in which a depletion region is formed by applying reverse-bias voltage to a pn junction formed by a pair of semiconductor layers whose conductive types are different from one another, when an electromagnetic wave of a light signal or the like is incident from the exterior, a pair of an electron and a positive hole is generated.
Further, this pair of the electron and the positive hole is multiplied by the avalanche phenomenon in the APD, and taken out as voltage or electric current to the exterior.
There are various ways of classifying APDs. When classifying structurally, there are a planar type and a mesa type, and when classifying by main carrier, there are a positive hole type and an electron type.
Here, a sequential mesa structure used regardless of the type of the main carrier will be described.
Generally, in order to aim for making the APD high-speed, the mesa type, not the planar type, is generally used as the shape of the APD.
This is for decreasing the electric capacity of the APD element itself in order to make the APD high-speed.
In order to increase the permissible light-receiving current as an APD element, there is the need to remove the bias of the light-receiving current density flowing through the interior of the mesa portion.
Therefore, in a mesa type APD element, the shape of the mesa must be made to be isotropic, namely, as shown in FIG. 9B, formed conically as viewed from the top surface of a substrate.
Moreover, in a mesa type APD element, when the shape of the mesa is formed to be conical, attention must be paid such that the crystallinity of the cross-section of the mesa is not damaged.
Therefore, in a mesa type APD element, when the shape of the mesa is fabricated, diffusive wet-etching by an etchant which is not anisotropic is necessary.
By applying this diffusive wet-etching, the sequential mesa shape, which is a shape (generally, conical) in which the mesa diameter (cross-sectional area) widens as it approaches the substrate, can be obtained.
Accordingly, the sequential mesa type APD is generally used for making the APD high-speed.
Further, as APDs using positive holes as the main carrier, there are an APD in which the above-described pn junction is formed by epitaxial growth, and an APD in which the pn junction is formed by Zn diffusion.
FIGS. 9A and 9B respectively show a cross-sectional view and an external perspective view of a sequential mesa type APD, in accordance with a prior art, which has a sequential mesa structure and in which positive holes are used as the main carrier and the pn junction is formed by epitaxial growth.
Hereinafter, on the basis of FIGS. 9A and 9B, the structure of the sequential mesa type APD according to the prior art will be described.
Namely, in the sequential mesa type APD according to the prior art, as shown in FIGS. 9A and 9B, an n-type buffer layer 2a, an n-type light absorbing layer 3a, an n-type electric field relaxation layer 4a, an n-type multiplying layer 5a, and a p-type contact layer 6b are successively formed by epitaxial growth by using an MOVPE (organometallic vapor phase epitaxial growth) method on an n-type semiconductor substrate 1a. Therefore, a conical sequential mesa portion 10 is formed by wet-etching from above.
Next, after a protective layer 7 is coated on the sequential mesa portion 10, a p electrode 8 contacting the p-type contact layer 6b is formed.
Further, at the both sides of the sequential mesa portion 10, an n electrode 9 is attached, via a protective layer 11, to another mesa portion formed for attaching electrodes.
As shown by the arrow in FIG. 9A, light incident on the APD from the bottom surface of the semiconductor substrate 1a penetrates through the semiconductor substrate 1a and the buffer layer 2a and is absorbed at the light absorbing layer 3a, so that a pair of an electron and a positive hole is generated.
Among the pair of the electron and the positive hole generated in this way, the electron moves to the n electrode 9 via the semiconductor substrate 1a, and the positive hole is multiplied at the multiplying layer 5a, and moves to the p electrode 8 via the contact layer 6b. 
In order to make the positive hole be the main carrier among the pair of the electron and the positive hole, a great number of the carriers of the light absorbing layer 3a must be electrons.
Namely, the conductive type of the light absorbing layer 3a must be n type.
Such a sequential mesa type APD uses a so-called SAM (Separate Absorption and Multiplication) structure, in which the multiplying layer 5a and the light absorbing layer 3a are separated by the electric field relaxation layer 4a such that a low electric field intensity is applied to the light absorbing layer 3a while a high electric field intensity is applied to the multiplying layer 5a. 
In this case, because the electric field intensity of the n-type light absorbing layer 3a is suppressed by the electric field relaxation layer 4a, the conductive type of the electric field relaxation layer 4a is the same n type as that of the light absorbing layer 3a. 
Because such a sequential mesa type APD has a function avalanche-multiplying the light exciting carrier, the crystallinity of the above-described layers is considered to be extremely important.
Note that, in such a sequential mesa type APD, the epitaxial growth itself of each layer can be carried out, in theory, on a semiconductor substrate which is any of an n-type semiconductor substrate, a p-type semiconductor substrate, or a semi-isolated semiconductor substrate.
As described above, in the sequential mesa type APD, when considering the fact that light-receiving current flows via the semiconductor substrate, the semiconductor substrate which is used must be an n-type or a p-type semiconductor substrate.
However, as shown in FIGS. 9A and 9B, because a dopant such as Sn, S or the like included in the semiconductor substrate 1a does not diffuse during the epitaxial growth, the n-type semiconductor substrate 1a is suitable as a substrate for the epitaxial growth of each semiconductor layer.
On the other hand, in the p-type semiconductor substrate, there are problems such as the Zn included in the semiconductor substrate diffuses during the epitaxial growth, there is the need to form a thicker buffer layer by epitaxial growth in order to prevent the Zn from diffusing, and because the n-type semiconductor substrate layer is formed by epitaxial growth after the p-type semiconductor substrate is formed by epitaxial growth, the time after the epitaxial growth of the p-type semiconductor layer becomes longer. Thus, diffusion of the Zn which is the dopant in the p-type semiconductor layer formed by the epitaxial growth easily arises.
Namely, the p-type semiconductor substrate having such problems is not generally suitable for a sequential mesa type APD in which crystallinity is considered to be extremely important.
Accordingly, it is preferable that the n-type semiconductor substrate 1a is used as the sequential mesa type APD in order to epitaxially grow a semiconductor layer having good quality crystallinity.
In this way, in order to obtain a good light-receiving characteristic in a sequential mesa type APD in which the positive holes are the main carrier and the pn junction is formed by epitaxial growth, the n-type light absorbing layer 3a and the n type field relaxation layer 4a are necessary, and the semiconductor substrate which is used is preferably the n-type semiconductor substrate 1a. 
Further, as described above, in a sequential mesa type APD in which the positive holes are the main carrier and the pn junction is formed by the epitaxial growth, as shown in FIGS. 9A and 9B, the p-type contact layer 6b is used in order to ensure an ohmic electrode in the p electrode 8.
At the time of epitaxial growth of the contact layer 6b, the contact layer 6b is doped to p type by using a p-type dopant such as Zn or the like.
Note that, in order to obtain the ohmic electrode, the p-type carrier density of the contact layer 6b is preferably set to be as high as possible, for example, about 5×1018 (cm−3) or more.
Note that the above-described MOVPE method or the like is mainly used as a growth method (manufacturing method) of the contact layer 6b. 
Further, due to the Zn which is the dopant of the contact layer 6b being diffused in the n-type electric field relaxation layer 4a, the conductive type of the multiplying layer 5a is made to be n type so that the appropriate internal electric field intensity distribution in the direction perpendicular to the n-type semiconductor substrate 1a is not destroyed.
Accordingly, the pn junction in the sequential mesa type APD is formed by the p-type contact layer 6b and the n-type multiplying layer 5a. 
Note that, in this case, the carrier density of the p-type contact layer 6b is particularly high as compared with the carrier density of the n-type multiplying layer 5a. 
Therefore, it is ideal that the sequential mesa type APD, in which the positive holes are used as the main carrier and the pn junction is formed by epitaxial growth, has the structure shown in FIGS. 9A and 9B.
Namely, because the sequential mesa type APD basically does not use a Zn diffusing process to be described later, there is the advantage that the manufacturing process (the process steps) can be simplified.
Further, because the sequential mesa type APD uses an n-type semiconductor in the electric field relaxation layer 4a which is difficult to be manufactured by a p-type semiconductor, there is the advantage that MOVPE, which can epitaxially grow at the wafer a semiconductor layer having high crystallinity, can be used as the method of manufacturing the sequential mesa type APD.
Next, a sequential mesa type APD, which has a sequential mesa structure and in which positive holes are used as the main carrier and the pn junction is formed by Zn diffusion, will be described.
The structure itself of such a sequential mesa type APD is the same as the structure of the sequential mesa type APD shown in FIGS. 9A and 9B.
As described above, in order to acquire excellent characteristics at the sequential mesa type APD in which the positive holes are the main carrier, the n-type light absorbing layer 3a and the n-type electric field relaxation layer 4a are necessary, and it is preferable to use the n-type semiconductor substrate 1a. This is also true in the case of a sequential mesa type APD in which the pn junction is formed by Zn diffusion, and in the case of the above-described sequential mesa type APD, in which the pn junction is formed by epitaxial growth.
Further, the contact layer 6b is made to be p type by diffusing Zn therein by a Zn diffusion method in order to ensure an ohmic electrode in the p electrode 8.
Note that, in order to obtain the ohmic electrode, the p-type carrier density of the contact layer 6b is preferably set to be as high as possible, for example, about 5×1018 (cm−3) or more.
Further, in the Zn diffusing method, by heating the Zn raw material and the wafer contained in a container filled with an inert gas atmosphere, the Zn is diffused from the surface of the wafer to the interior of the wafer.
At this time, in order to carry out sufficient Zn diffusion, there is the need to control the gas pressure of the inert gas atmosphere so as to maintain a relatively high value by using an exclusively-used controller, and there is the problem that the manufacturing process (process steps) is complicated.
The Zn diffused in this way remains in the contact layer 6b, and the p-type carrier density is enhanced to a degree at which an ohmic electrode can be obtained, for example, to 5×1018 (cm−3) or more.
Note that, at this time, because the Zn is not diffused in the multiplying layer 5a, the conductive type of the multiplying layer 5a is maintained as n type.
In accordance therewith, the pn junction is formed by the p-type contact layer 6b, in which the p-type carrier density is increased by Zn diffusion, and the n-type multiplying layer 5a. 
As a result, also in the case of a sequential mesa type APD in which positive holes are used the main carrier and the pn junction is formed by Zn diffusion, the structure shown in FIGS. 9A and 9B is ideal.
Further, the sequential mesa type APD in which the pn junction is formed by Zn diffusion has the advantage that the desired pn junction can be formed by appropriately setting the diffusing conditions of the Zn.
Further, the sequential mesa type APD in which the pn junction is formed by Zn diffusion also has the advantage that, because an n-type semiconductor is used as the electric field relaxation layer 4a which is difficult to fabricate by a p-type semiconductor, the MOVPE method, by which a highly crystalline semiconductor layer can be epitaxially grown on the wafer, can be used as the manufacturing method.
On the other hand, because the sequential mesa type APD uses a Zn diffusing process, the sequential mesa type APD has the drawback that the manufacturing process (process steps) is complicated due to the above-described reasons.
Next, the sequential mesa type APD, which has a sequential mesa structure and in which electrons are used as the main carrier and the pn junction is formed by epitaxial growth, will be described.
FIG. 10 shows a cross-sectional view of the sequential mesa type APD which has a sequential mesa structure and in which electrons are used as the main carrier and the pn junction is formed by epitaxial growth.
Note that, in this FIG. 10, portions which are the same as those of the sequential mesa type APD shown in FIG. 9A are denoted by the same reference numerals.
Further, an external perspective view of the sequential mesa type APD, which is shown in FIG. 10 and in which electrons are used as the main carrier and the pn junction is formed by epitaxial growth, is the same as in FIG. 9B, and thus, illustration is omitted.
Namely, as shown in FIG. 10, in the sequential mesa type APD in which electrons are used as the main carrier and the pn junction is formed by epitaxial growth, after the n-type buffer layer 2a, the n-type multiplying layer 5a, the p-type electric field relaxation layer 4b, the p type light absorbing layer 3b, a p-type window layer 13b, and the p-type contact layer 6b are successively formed by epitaxial growth on the n-type semiconductor substrate 1a by using an epitaxial growth method, the conical sequential mesa portion 10 is formed by wet-etching from above.
Further, after the protective layer 7 is coated on the sequential mesa portion 10, the p electrode 8 contacting the p-type contact layer 6b is formed.
Further, on the both sides of the sequential mesa portion 10, the n electrodes 9 are attached, via the protective layer 11, to another mesa portion formed for attaching electrodes.
In such a sequential mesa type APD in which electrons are the main carrier, as shown by the arrow in FIG. 10, light incident from the bottom surface of the semiconductor substrate 1a penetrates through the semiconductor substrate 1a, the buffer layer 2a, the multiplying layer 5a, and the electric field relaxation layer 4b and is absorbed at the light absorbing layer 3b, so that a pair of an electron and a positive hole is generated.
Among the pair of the electron and the positive hole generated in this way, the electron is multiplied at the multiplying layer 5a and moves to the n electrode 9 via the n-type semiconductor substrate 1a, and the positive hole moves to the p electrode 8 via the contact layer 6b. 
In order to make the electron be the main carrier among the pair of the electron and the positive hole, a great number of carriers of the light absorbing layer 3b must be positive holes.
Namely, in this case, the conductive type of the light absorbing layer 3b must be p type.
In such a sequential mesa type APD in which electrons are the main carrier, the above-described SAM structure, in which the multiplying layer 5a and the light absorbing layer 3b are separated by the electric field relaxation layer 4b such that a low electric field intensity is applied to the light absorbing layer 3b while a high electric field intensity is applied to the multiplying layer 5a, is used.
In this case, because the electric field intensity of the p type light absorbing layer 3b is suppressed by the electric field relaxation layer 4b, the conductive type of the electric field relaxation layer 4b is p type which is the same as that of the light absorbing layer 3b. 
Further, because such a sequential mesa type APD in which electrons are the main carrier has a function avalanche-multiplying the light exciting carrier, the crystallinity of the above-described layers is considered to be extremely important.
In order to obtain excellent crystallinity of each semiconductor layer, for the same reasons as in the case of the sequential mesa type APD described in FIGS. 9A and 9B in which positive holes are the main carrier, the semiconductor substrate which is used is preferably the n-type semiconductor substrate 1a. 
Moreover, in order to improve the accuracy of the electric field intensity distribution in the sequential mesa portion 10 in the direction perpendicular to the semiconductor substrate 1a, because the pn junction is preferably formed between the p-type electric field relaxation layer 4b and the multiplying layer 5a, the multiplying layer 5a is n type.
Such a formed position of the pn junction is also preferable for making estimation of the amount of decrease in the electric field intensity in the multiplying layer 5a be unnecessary.
Accordingly, in the sequential mesa type APD in which electrons are the main carrier, the pn junction is formed by the p-type electric field relaxation layer 4b and the multiplying layer 5a. 
In this way, in order to obtain excellent light-receiving characteristics in a sequential mesa type APD in which the electrons are the main carrier and the pn junction is formed by epitaxial growth, the p type light absorbing layer 3b, the p-type electric field relaxation layer 4b, and the n-type multiplying layer 5a are necessary, and the semiconductor substrate which is used is preferably the n-type semiconductor substrate 1a. 
In such a sequential mesa-type APD, the window layer 13b also is necessary in order to prevent the electrons which are a light exciting carrier from diffusing/moving to the contact layer 6b. 
Note that GS-MBE (gas-molecule beam epitaxy), MBE (molecule beam epitaxy), and the like are mainly used as the epitaxial growth method.
Further, in order to ensure the ohmic electrode of the p electrode 8, the conductive type of the contact layer 6b is p type.
Moreover, at the time of epitaxial growth, the contact layer 6b is doped to a p type by using a p-type dopant such as Be or the like.
Note that, in order to obtain the ohmic electrode, the p-type carrier density of the contact layer 6b is preferably set to be as high as possible, for example, about 5×1018 (cm−3) or more.
Accordingly, in a sequential mesa type APD in which electrons are the main carrier, the structure shown in FIG. 10 is ideal, and because electrons having a light effective mass are the main carrier, there is the feature that it is advantageous with respect to the point of high-speed performance.
However, in the APDs having the sequential mesa structures shown in FIGS. 9A, 9B and FIG. 10, there are still the following problems which must be improved.
Firstly, in the sequential mesa type APD in which positive holes are the main carrier, or also in the sequential mesa type APD in which electrons are the main carrier, there is the problem that, in each semiconductor layer forming the sequential mesa portion 10, except for the case of selectively diffusing Zn at a specific portion in the surface parallel to the semiconductor substrate, it is difficult for the in-surface distribution of electric field intensity in a surface parallel to the semiconductor substrate to concentrate at the central portion of the mesa by only the epitaxial growth process.
FIG. 3 shows measured results of the light-receiving sensitivity distribution characteristic of a sequential mesa type APD whose light-receiving diameter is 40 μm.
Concretely, FIG. 3 shows measured values of light-receiving current (μA) obtained between the p electrode 6 and the n electrode 9 at each position (μm) in a case in which the irradiating position of an extremely thin light beam is successively moved within the aforementioned range of 40 μm.
In FIG. 3, characteristic B shows the light-receiving sensitivity distribution characteristic of the sequential mesa type APD as shown in FIGS. 9A and 9B.
As illustrated, characteristic B is a double-peaked characteristic in which the light-receiving current at the peripheral portion of the mesa shown by the positions −20 μm, +20 μm from the central position (0) is larger than the light-receiving current at the central portion of the mesa.
A sequential mesa type APD whose light-receiving characteristic is a double-peaked characteristic in this way has the problem that it is difficult to align the optical axes at the time of actual use when made into a module, and the yield of the modularization deteriorates. Because alignment of the optical axes must be carried out at the central portion of the mesa at which the light-receiving current is smaller than that of the peripheral portion of the mesa, a sufficient light-receiving characteristic cannot be exhibited. In addition, it is difficult to realize high sensitization by keeping to a minimum the effects of the dark current and noise contained in the light-receiving signal relating to the problem of crystallinity described later, and to decrease the fabricating costs of modularization.
Hereinafter, reasons why these problems arise will be described.
Because the APD shown in FIGS. 9A and 9B is a sequential mesa type structure, the more the electric field intensity increases, the more the carrier of the positive holes or the electrons is multiplied.
Accordingly, the magnitude of the light-receiving current shows the magnitude of the electric field intensity at the pn junction.
It can be said that the electric field intensity at the periphery of the mesa is higher and the electric field intensity at the central portion of the mesa is low in the sequential mesa type APD shown in FIGS. 9A and 9B.
FIG. 11 shows the way of broadening (width) of the depletion region (depletion layer) by built-in potential from the pn junction in the sequential mesa type APD shown in FIGS. 9A and 9B in which positive holes are used as the main carrier.
Note that, as described above, because the carrier density of the p-type contact layer 6b forming the pn junction is higher than the carrier density of the multiplying layer 5a, the majority of the depletion region (depletion layer) is formed at the semiconductor substrate 1a side of the pn junction.
As shown in FIG. 11, because this APD has a sequential mesa structure, the ratio of the cross-sectional area showing the depletion region of the p-type contact layer 6b structuring the pn junction and the cross-sectional area showing the depletion region of the n-type multiplying layer 5a greatly differs at the central portion of the mesa and at the peripheral portion of the mesa.
Here, considering from the standpoint of depleting the pn junction portion, because the APD has a sequential mesa structure, at the vicinity of the periphery of the mesa, there is a state in which the carrier density of the multiplying layer 5a is substantially higher than at the central portion of the mesa.
In contrast, at the contact layer 6b, conversely, there is a state in which the carrier density is weak. However, because the carrier density is originally high at the contact layer 6b, even if it is in a state in which the carrier density is substantially weak, the effect is small.
As a result, in the sequential mesa type APD, the way of broadening (width) of the depletion region is shorter (narrower) than the way of broadening (width) of the central portion.
Namely, it can be understood that the electric field intensity at the peripheral portion of the mesa is higher than that at the central portion of the mesa in the sequential mesa type APD.
FIG. 12 shows the way of broadening (width) of the depletion region (depletion layer) by built-in potential from the pn junction portion in the sequential mesa type APD as shown in FIG. 10 in which electrons are used as the main carrier.
In this sequential mesa type APD, the carrier density of the p-type electric field relaxation layer 4b forming the pn junction is higher than the carrier density of the multiplying layer 5a. Thus, as shown in FIG. 12, in accordance with the principles of charge neutrality, the way of broadening (width) of the depletion region at the vicinity of the periphery of the mesa is shorter (narrower) than the way of broadening (width) of the central portion.
Namely, in the sequential mesa type APD, the electric field intensity at the peripheral portion of the mesa is higher than that at the central portion of the mesa.
The reason for this is that, in the sequential mesa type APD, it is difficult for the in-surface distribution of field intensity in a surface parallel to the semiconductor substrate to concentrate at the central portion of the mesa by only the epitaxial growth process, so that there is a double-peaked characteristic in which the light-receiving current at the peripheral portion of the mesa is greater than the light-receiving current at the central portion of the mesa.
In this way, in the sequential mesa type APD in which positive holes or electrons are used as the main carrier and the pn junction is formed by epitaxial growth, the way of broadening (width) of the depletion region at the vicinity of the periphery of the mesa is shorter (narrower) that at the central portion, and the electric field intensity at the peripheral portion of the mesa is higher than at the central portion of the mesa.
Here, the relationship between the crystallinity and the light-receiving characteristic of the sequential mesa type APD will be described.
As described above, a sequential mesa type APD of this type, the light-receiving current is multiplied by an avalanche multiplying function.
Further, the noise at the time of the avalanche multiplying function greatly depends on the crystallinity of the sequential mesa type APD.
Accordingly, even among light-receiving elements in which crystallinity is considered to be important, in particular, the crystallinity of a sequential mesa type APD is important.
In a sequential mesa type APD, a mesa side surface 10a formed by mesa-etching is provided at the peripheral portion of the mesa of a sequential mesa portion 10.
Generally, the mesa side surface 10a has a great number of crystal defects as compared with the interior portion of the mesa.
Further, the crystal defects adversely affect the consideration of solutions for decreasing dark current in the sequential mesa type APD, decreasing noise, high sensitization, and modularization.
Namely, in a sequential mesa type APD in which positive holes or electrons are used as the main carrier and the pn junction is formed by epitaxial growth, as shown by characteristic B of FIG. 3, when the light-receiving characteristic of the sequential mesa type APD is dominant at the peripheral portion of the mesa, the good crystallinity which the central portion of the mesa has is not reflected in the light-receiving characteristic of the entire sequential mesa type APD. As a result, it is a cause for the light-receiving characteristic of the entire sequential mesa type APD to deteriorate, and for it to be difficult to align optical axes at the time of making the APD a module, and for the yield of modularization to be poor, and for the fabricating costs of modularization to increase.